To unlock HK-51 on a low level character, it is a 2 step process
1. Unlock him via the mission chain on a level 50 character.
2. Purchase him via credits or cartel coins on the Legacy Character Perks page.
Note: it is a Legacy perk, if you start a new legacy on a different server, then you will have to get the new character to level 50 and unlock HK-51 via the mission chain.

As a subscriber I can tell you it is NOT automatic that you get him on all new characters.

Is he worth buying for a low level characters? Hell yea. You will need to get some level appropriate gear for him since he comes with lvl 50 stuff (untill you are 50, it is "broken" on him) a couple thousand at the GTN will work, but a decent blaster/sniper rifle and a couple other items, and frankly he is overpowered until around level 30. At which point things start balancing out, but he is still useful through to level 50.

"The Dark Side of the Force is a pathway to many abilities some consider to be unnatural."